This invention relates to a process of performing measurements of cell metabolism and cell growth under different conditions.
Presently, in the field of rapid detection or measurement of microbial activity, and in the rapid detection or measurement of susceptibility of microorganisms to molecules, agents, or other cells, there is available a wide variety of apparatus and processes. Generally, these are based on the measurement of the physical characteristics of many viable cells (microorganisms), the growth of individual cells into many viable cells, or the growth and metabolism of many viable cells, or the composition of many cells. Examples of measurements of the physical characteristics of many cells include light attenuation (turbidity), light scattering and viscosity. Examples of the measurement of the growth of individual cells into many cells includes culturation on a petri dish or other gel surface or culturation within a gel contained in an optically transparent capillary. Examples of the measurement of the growth and metabolism of many viable cells includes measurement of pH changes of the medium containing the viable cells, measurement of non-radioactive accumulated volatile metabolites in the headspace over a sample, measurement of heat production, or measurement of electrical conductivity of the medium containing the viable cells. An example of staining includes the measurement of fluorescent labeled antibody, the antibody binding to species specific surface antigens on the cells. An example of the measurement of the composition of many cells is pyrolysis/mass spectrometry, which method is destructive in that it rapidly fragments cells under conditions of high temperature.
While the present procedures are highly satisfactory in many respects, two important problems exist. First, procedures which measure the growth of individual cells into many cells, such as culturation in a gel, have the highly desirable feature that they do provide an enumeration or count of the original number of cells present in a sample, but have the undesirable feature that they require a long incubation time, often 12 to 48 hours, to obtain a measurable colony from an original cell. This long incubation is also required for the measurement of the physical characteristics of many cells for the composition of many cells since it is generally not possible to obtain a sufficiently pure sample, free of debris and other cell types, without culturation. Further, measurement of the metabolism of many cells also requires a lengthy incubation time if the cell density in the sample is initially small. Second, procedures which measure the physical characteristics of many cells, the composition of many cells or the growth and metabolism of many cells do not provide an enumeration or count of the cells present, since there is often considerable variability between the individual cells. An important example is a variable and unknown lag time which often occurs when measurement of growth and metabolism of many cells is employed with a sample containing a small initial number of cells. In this case, a variable lag time can lead to the inability to correlate the subsequently measured growth and metabolism of many cells with enumeration or count of the initial cells. A particularly undesirable occurrence is the presence of initially stressed or injured cells which have a long, but a priori unknown, lag time, since the result of the measurement in this case is a false negative.
Measurement of individual microbiologically active entities such as individual enzyme molecules has been proposed by Rotman, Proceedings National Academy of Sciences, Vol. 47, Pages 1981-1991, (1961), who discloses the formation of water droplets in oil which droplets contain a small number of enzymes. However, this procedure is very tedious, difficult to replicate and the enzymes are susceptible to migration from the droplets to the oil-water interface.
It would be desirable to provide a means for measuring cell metabolism or growth to provide a base count and for exposing cells from the same specimen as those from which the base count is made to one or a variety of compositions which affect cell metabolism or growth which is capable of permitting determination of the extent of the effect from the composition within a short time, e.g. 1-10 hours. Such a process would be useful in determining the susceptibility of cells to various antibiotics by exposing a sample of the cells of various antibiotics. Such a process would be useful in determining the mode of treatment of a patient who is the source of the cells, e.g. by treatment with an antibiotic which exhibits a desired effect on cell metabolism or growth.